An optical amplifier may be arranged to perform a particular system/amplifier function in response to a change in particular stimuli. For example, an optical amplifier may be arranged to change the level of its optical pump power in response to a change in the power level of an incoming optical signal, which may be due to a change in the number of optical channels carried by the incoming signal or due to a change in span loss.
A change in an incoming optical signal may also be due to an optical fiber nonlinearity. One such nonlinearity is commonly referred to as Raman gain. The Raman gain, or effect, becomes particularly troublesome when an appreciable level of optical power distributed over a certain range of wavelengths is pumped into an optical fiber. In that instance, the Raman gain is tilted in favor of the channels having the longer wavelengths, which degrades the Signal-to-Noise Ratio (SNR) of the signals in the lower wavelength channels and thus seriously degrades their performance. Moreover, the power levels of higher wavelength channels may increase, thereby making those channels more susceptible to non-linearity problems. Raman gain may be dealt with at an amplifier by "tilting" the outputted optical signal in a direction opposite to the Raman gain, as disclosed in copending U.S. patent application of Ser. No. 09/265,943 filed Mar. 8, 1999, which is hereby incorporated by reference.
A problem arises when a downstream optical amplifier in an optical transmission system having a plurality of optical amplifiers disposed along the optical transmission path responds independently in the described manner to a change in the power level of an incoming optical signal. More specifically, a downstream optical amplifier may tilt its output signal in the wrong direction, or exaggerate the tilt, if it performs its tilt adjustment before an upstream optical amplifier has completed its tilt adjustment. Also, the optical transmission system may not stabilize with respect to a change in the input signal, or a change in some other nonlinearity, if each optical amplifier in the transmission path is allowed to proceed independently.